The present invention relates to ice thermal storage systems and more particularly teaches an improved method and apparatus for measuring the amount of ice present on an ice thermal storage heat exchanger, such as a tubular coil assembly.
Ice thermal storage equipment of the type which forms ice during off peak energy periods and then makes the ice available as a supply of cold or low temperature fluid for space conditioning, and the like, is known in the art. The ice thermal storage equipment may be broadly classified as internal melt systems and external melt systems. One form of thermal storage equipment and external melt system transfers a coolant liquid, such as brine or ethylene glycol solution, through a coil assembly immersed in a tank of storage liquid to be frozen, which liquid may for example be water. The coil assembly is usually a serpentine configuration of bent tubing with multiple tube runs nested in the tank and storage liquid. Multiple coil assemblies are usually packed in parallel within the tank. The coil assemblies are connected between inlet and outlet headers for receipt and discharge of the coolant liquid from and to one or more heat exchangers or chillers, which cool the coolant fluid during the ice production cycle. Although the use of the coolant liquid has been noted as a brine solution, it is noted that the coolant liquid could be a refrigerant, such as R-22 or ammonia.
During the ice-production cycle, coolant liquid, such as brine, at a temperature below the solidification point of the storage liquid within the pool is continuously produced by mechanical refrigeration or other means in a heat exchanger, sometimes referred to as a chiller. The low-temperature coolant is transferred to an inlet header, through the coil assemblies and discharged from the outlet header for return to the chiller. The storage liquid in the tank is frozen on the tube outer walls in the form of surrounding envelopes and gradually develops as a substantial thickness of frozen liquid, usually ice. There is a volume of the storage liquid in the tank, which remains as a liquid. During recovery of the stored thermal energy, the chilled storage liquid is withdrawn from the tank and communicated to a downstream cooling coil or heat exchanger for use in cooling operations, such as air conditioning or food processing. Thereafter, the spent or warmed storage liquid is returned to the tank to be cooled and further used for cooling operations.
Efforts have been made to measure or quantify the degree of freezing of the liquid in the tank. The underlying reasons for the desire to quantify the frozen liquid is to know the amount of stored cooling capacity that exists in the tank. One method of ice build up measurement positions a coil on springs and employs load cells to sense the uplifting force of the coils, which are restrained from vertical movement. The intent of this apparatus is to relate the uplifting force to the quantity of ice on the coil. The precise structure and schematic drawings are not available in a published format for this load cell system.
Current means used to measure or monitor the ice build up on the coil assemblies in the tank liquid pool have included visual inspection of the ice at the surface, which is not considered to be efficient or measurable. Another ice-measurement method uses a fluid level monitor, which operates on the principle that a pound of ice occupies more volume than a pound of water. These devices are not relied upon in cases where the hydraulic system is not a closed loop. In addition, ice thickness measurements are provided in an external-melt system by utilization of electronic probes noting the change in conductivity on the tubes as the ice develops. However, the probes used in this method have proven to be fragile.